1. Field of the Invention
The present invention relates to a new liquid crystal compound, a liquid crystal composite including the compound and a liquid crystal device utilizing the composite, and more particularly to a new liquid crystal composite having improved responsive characteristic to an electric field and a liquid crystal device utilizing the composite and for use in a liquid crystal display device, a liquid crystal-light shutter and the like which use the composite.
2. Description of the Prior Art
A liquid crystal has been conventionally applied to various kinds of fields as an electro-optic device. Most liquid crystals which are in practical use at present use Twisted Nematic (TN) liquid crystals, which are described in, for example, M. Schadt and W. Helfrich, "Voltage Dependent Optical Activity of a Twisted Nematic Liquid Crystal", Applied Physics Letters, Vol 18, No.4 (Feb. 15, 1971), pp.127-128.
These devices are based on dielectric alignment effects of the liquid crystal, in which by virtue of dielectric anisotropy, the average molecular long axis takes up a preferred orientation in an applied electric field. It is said that the minimum unit usable for the optical response speed of these devices is a millisecond, and this speed is too low for many potential applications. On the other hand, a simple matrix drive method is the most advantageous for an application of the liquid crystal to a wide flat display in consideration of price, productivity and so on. In the simple matrix method, scanning electrodes and signal electrodes are structured in a matrix. In order to drive the electrodes, a time-sharing drive method is adopted in which address signals are selectively applied to the scanning electrodes sequentially and cyclically, and predetermined information signals are selectively applied to the signal electrodes in parallel and in synchronization with the address signals.
However, when the above TN liquid crystal is applied to a device used in such a drive method, the electric field is finitely applied to a region where the scanning electrodes are selected and the signal electrodes are not selected, or a region where the scanning electrodes are not selected and the signal electrodes are selected (what is called a half-selected point).
If the difference between the voltage applied to the selected point and the voltage applied to the half-selected point is sufficiently large and a voltage threshold value required to align the liquid crystal molecules perpendicular to the electric field can be set at a voltage value between the voltage to the selected point and the half-selected point, the display device operates normally. However, as the number (N) of scanning lines is increased, the time (duty ratio) when an effective electric field is applied to one selected point during a scanning for an entire screen (1 frame) is decreased in the ratio of 1/N.
Therefore, when scanning operations are repeated, the difference as an effective value between the voltage to the selected point and the voltage to the non-selected point is increased as the number of scanning lines is increased. As a result, the lowering of an image contrast and cross talk are inevitable.
Such phenomena are substantially inevitable problems caused when a liquid crystal without bistability (its safe state is the state in which liquid crystal molecules are aligned horizontally to the electrode plane and the molecules are aligned perpendicularly only while the electric field is effectively applied) is driven by using the time cumulative effect (that is, when scanning operations are repeated).
Although the voltage averaging method, the dual frequency drive method, the multiplexing matrix method and so on have already been suggested in order to solve these problems, none of these methods is sufficient, and the improvements of a large screen and high density of display devices have reached the limit in the present since the number of scanning lines cannot be sufficiently increased.
In order to obviate such defects of a conventional liquid crystal device, the use of a liquid crystal device having bistability is suggested by Clark and Lagerwall (see Japanese Unexamined Patent No. 56-107216, U.S. Pat. No. 4,367,924 and so on).
As a bistable liquid crystal, a ferroelectric liquid crystal including a chiral smectic C phase (ScC* phase) or a H phase (SmH* phase) is used in general.
The ferroelectric liquid crystal has a bistable state which is composed of first and second optical stable states with respect to the electric field, and is different from the optical modulation device used in the above-mentioned TN liquid crystal. For example, the liquid crystal is aligned in the first optical stable state with respect to one electric field vector, and aligned in the second optical stable state with respect to the other electric field vector. Furthermore, this type of liquid crystal selects one of the above two stable states in response to the electric field to be applied and maintains the selected state when the electric field is not applied, that is, has bistability.
The ferroelectric liquid crystal has the excellent characteristic of having a high responsibility besides the bistability. This is because the spontaneous polarization, which the ferroelectric liquid crystal has, and the applied electric field directly operate to induce the transition of the alignment state, and the response speed of the ferroelectric liquid crystal is 3-4 orders higher than the response speed in the operation of the anisotropy of the dielectric constant and the electric field.
Thus, the ferroelectric liquid crystal latently has extremely excellent properties, and it is possible to essentially improve most of the above-described problems of the TN liquid crystal by utilizing such properties. In particular, applications of the ferroelectric liquid crystal to a high-speed optical shutter and a display on a wide screen with high density can be hoped. Although broad studies of liquid crystal materials having ferroelectricity have been made, it is difficult to say that the ferroelectric liquid crystal materials., which have been developed up to now, have sufficient properties including a low temperature operation property and a high-speed responsibility to be used in a liquid crystal device.
The relationship among the response time .tau., the amount Ps of the spontaneous polarization and the viscosity .eta. is shown by the following equation [II]: ##EQU1## (where E represents an applied electric field). Therefore, in order to increase the response speed, the following methods are used:
(a) to increase the amount Ps of the spontaneous polarization; PA1 (b) to lower the viscosity .eta.; and PA1 (c) to decrease the applied electric field E.
However, since the drive is performed by an IC or the like, the applied electric field has the upper limit and is desirable to be as low as possible. Therefore, actually, it is necessary to lower the viscosity .eta. or to increase the amount Ps of the spontaneous polarization.